The present invention relates to a method and apparatus for melting scrap in which the melting heat is generated by the partial combustion of coal dust and coke dust together with oxygen in a melting chamber and the remaining energy of the gas generated by the partial combustion is used to preheat the scrap to the extent that separate utilization of the remaining gas is not required. The melting unit is composed of a vertical shaft for preheating the scrap and a melting chamber disposed therebelow.
Methods for melting scrap in which carbon carriers are used to generate the melting heat are known. For example, it has been proposed to melt scrap in cupola furnaces as they are used in the form of blast furnaces for the production of cast iron. In such furnaces, the metal material is melted with the addition of slag forming additives (limestone) and metallurgical coke as the energy carrier. The combustion air, or "wind" is compressed by a blower and blown through nozzles into the furnace shaft through nozzles. A distinction is made between "cold wind" and "hot wind" operation, depending on whether the combustion air is introduced cold or preheated.
The melting process takes place as follows: charges introduced through charge openings fill the furnace shaft approximately to the level of a charging platform. Ascending hot furnace gases heat the charge, which gradually slides down into the furnace shaft as it melts. After reaching the melt zone (300 to 400 mm above the nozzles), the iron begins to flow and drops through a bed of coke, i.e. into a column of filled-in coke which supports the charge and which extends from the bottom to the melt zone. This causes the iron to be highly carbonated.
New developments are the "cupola furnace without lining" in which the shaft is composed of a water-sprinkled metal body without refractory lining and the "cupola furnace without coke" which is selectively operated with gas or oil. Instead of the supporting coke frame, the furnace has a water-cooled grate and a structure of refractory balls above it. Additionally, there are cold wind cupola furnaces with secondary wind operation in which air is blown into the furnace shaft to partially post-combust combustible gas components. In these cases, partial back reactions of the developing carbon dioxide with the coke to form carbon monoxide (Boudouard reaction) cannot be prevented.
It has also been proposed to melt scrap by using a hearth furnace having water cooled wall elements as the refining vessel. In this process, which is called the EOF process, oxygen is blown through nozzles disposed on the side of a hearth furnace below the surface of the bath, and oil and/or coal dust and oxygen are blown through nozzles disposed in the sides above the bath surface. Enriched hot wind is introduced through blow molds disposed above the oil/coal dust/oxygen burners and the exhaust gas is completely post-combusted in the furnace vessel. The hot furnace exhaust gases are conducted through a scrap preheating system disposed above the furnace and are then fed into a recuperator to preheat the blown air. The scrap is preheated to a maximum of 800.degree. C. and is fed into the hearth furnace in discontinuous charges.
The drawbacks of this method are the complicated mechanical structure of the high temperature scrap preheater and the discontinuous manner of charging. The exhaust gas temperatures in this process fluctuate over time and thus make further utilization of the exhaust gas heat more difficult.
To increase the melting output of steel mill converters, methods have recently been developed in which coal dust and oxygen are blown into the metal bath through special nozzles disposed in the bottom of the converter. The heat released during the partial combustion of the carbon to carbon monoxide is utilized to melt scrap or sponge iron. Examples for such methods are the so-called COIN process and the KMS method. In the KMS method, oxygen is additionally blown onto the bath surface and the exhaust gas is partially post-combusted.
A significant drawback of these processes is the poor gas utilization. For the economical operation of such systems, secondary utilization of the exhaust gases is absolutely necessary.